This invention relates to erbium-doped fibers for photonic devices, and more particularly to erbium-doped fiber amplifiers (EDFA) operating in the L-band.
Optical transmission systems employ Wavelength Division Multiplexing (WDM) to increase information handling of an optical fiber transmission line, typically a long haul transmission line. Early WDM systems operated with a relatively narrow wavelength bandwidth, centered around 1550 nanometers, e.g. 1530-1565 nanometers, often referred to as the C-band. This is the wavelength region where standard silica based optical fibers have optimally low absorption.
In most WDM systems there is a trade-off between the number of channels the system accommodates and the channel separation. Both goals favor a wide operating spectrum, i.e. a wide range of operating wavelengths.
Recently, systems have been designed that extend the effective operating wavelength range well above the C-band transmission band. In terms of wavelength, the new band, referred to as the L-band, is variously defined, but for the purpose of this description is 1570-1610 nanometers. Use of these added wavelengths substantially extends the capacity of WDM systems. However, there is an ongoing effort to further extend the effective operating wavelength window, even beyond 1610 nanometers. These efforts are currently focused on the so-called xe2x80x9cextended L-bandxe2x80x9d, from 1570 to above 1610 nm, for example to 1620 nm.
Rare-earth-doped fiber amplifiers have found widespread use as amplifiers in WDM communication systems. Typically these amplifiers are erbium doped fiber amplifiers (EDFA). They are easily integrated with long haul fiber cables and can be pumped conveniently using inexpensive multi-mode lasers, such as GaAlAs, with high power, single mode outputs. They can also be made with relatively wide gain bandwidth to achieve some of the goals just mentioned.
WDM systems may also employ dispersion shifted fiber (DSF) which carries more wavelengths in the L-band than in the C-band. Using these advanced design components, EDFAs and DSF transmission lines, dense WDM (DWDM) systems have been developed that are capable of transmitting 40xc3x9710 Gbit/s wavelengths in the L-band, providing a 16,000% increase in network capacity. Translated into practical information handling capacity, this system is able to transmit simultaneously the data contained on more than 80 CD ROMs each second.
In WDM systems, it is important to have uniform gain over the entire WDM wavelength band. This objective becomes more difficult to reach as the operating wavelength range is extended to longer wavelengths. Modest non-uniformities in the gain curve may be filtered out by chopping the high gain peaks. However, typical gain curves have curvature over substantial portions of the wavelength band so that chopping the high gain portion may waste a substantial amount of signal. Thus an optical amplifier with relatively uniform, and relatively flat, gain over the L-band, including the extended L-band, would represent an important technological advance in DWDM system design.
A rare-earth doped fiber and fiber amplifier have been developed for operation in the extended L-band. The optical fiber used for this invention incorporates erbium for the basic light amplification function, phosphorus to selectively enhance gain at the target wavelengths in the extended L-band, germanium to provide a high numerical aperture (NA) and aluminum to solubilize the other additives. The amount of aluminum is controlled to below that of phosphorus, and generally below 4%, to maintain flatness of the gain curve. The actual functions of these additives are partly interchangeable, and the precise role of each has not been thoroughly investigated. But the effect of the additives in the combinations described below has been established.
Optical fibers produced according to the invention exhibit a flat gain profile that extends out to at least 1620 nanometers, thus broadening the bandwidth of the L-band by more than 20%